In response to the demand for semiconductor chip packages having high lead counts and small footprints, ball grid array (BGA), "flip chip" and, more recently, "chip scale" packages have been developed. These packages are surface mounted to an external printed circuit board using an array of solder balls or similar electrode bumps, instead of metal leads. A BGA package utilizes a printed circuit board type substrate instead of a lead frame. In a typical BGA package, the semiconductor chip is mounted to the top surface of a substrate. The chip is wire bonded to electrical traces in the substrate. The chip is then overmolded with an encapsulating material. Solder balls are bonded to the electrical traces in the bottom surface of the substrate. The solder balls serve as the external electrodes for surface mounting on a printed circuit board.
Flip chip packages are similar to BGA packages, except that the solder balls are attached directly to the bond pads or electrical traces formed in the surface of the chip resulting in a more compact package. Due to the absence of an encapsulating material, flip chip packages are quite fragile and require careful assembly and handling techniques. Chip scale packages are being developed to combine the durability of the BGA packages and the small size of the flip chip packages. In a chip scale package, solder balls are attached to electrical traces in the surface of the chip or directly to the bond pads through openings in the encapsulating material. The electrical traces connect the solder balls, which form external electrodes bumps, to the bond pads in the chip. The chip scale package need only be slightly larger then the bare chip because the electrode bumps are formed on the surface of the chip.
It is desirable that the new smaller semiconductor chip packages be adapted for use in standard formats developed for larger chips. However, for both the flip chip and chip scale packages, in which the electrode bumps are attached directly to contacts on the surface of the chip, the electrode bump array must be reconfigured each time the chip is made smaller. Hence, the corresponding contacts on the printed circuit board to which the package is ultimately assembled (the chip "footprint"), must also be reconfigured. In addition, there remains a need for a semiconductor chip package that combines the small size of chip scale packages with the durability and economies of conventional lead frame assemblies and encapsulating techniques currently used to form molded plastic packages.